(1) Field of the Invention
The present invention generally relates to a bolt-fastened structure, and more particularly to a bolt-fastened structure of a supporting member which fastens a wheel of an automotive vehicle to an axle hub by tightening nuts to hub bolts fitted in through holes of the axle hub.
(2) Description of the Related Art
A bolt-fastened structure of an axle hub of an automotive vehicle which is used to fasten a disc wheel to the axle hub by using bolts and nuts, is known. Japanese Laid-Open Utility Model Application No.62-31502 discloses a bolt-fastened structure of this type.
In the bolt-fastened structure of the above-mentioned publication, hub bolts are fitted in through-holes of the axle hub, and nuts are tightened to the hub bolts to fasten the disc wheel to the axle hub. In this bolt-fastened structure, each hub bolt has a serrated neck portion, the serrated neck portion being formed on the outer periphery of the hub bolt below a head of the hub bolt. When the hub bolts are fitted in the through-holes of the axle hub, the serrated neck portion of each hub bolt serves to prevent a rotation of the hub bolt relative to the axle hub by a tightening torque needed to tighten the nut to the hub bolt.
If a self-locking resistance of the serrated neck portion of the hub bolt to the axle hub is not greater than the tightening torque, the hub bolt is loosely rotated, together with the nut, relative to the axle hub by the tightening torque on the nut, and it is difficult to firmly fasten the disc wheel to the axle hub. In the structure of the above publication, since the hub bolt has the serrated neck portion, the hub bolt can be firmly fitted to the axle hub, and the nut can be tightened to the hub bolt effectively to secure the disc wheel to the axle hub.
On the other hand, it is desired that a maintenance operation to replace the hub bolts on the axle hub of the automotive vehicle can be easily performed in the field. One of measures to facilitate the maintenance operation is to widen an insertion space that the hub bolts can be inserted into the through-holes of the axle hub without interfering to other parts of the vehicle surrounding the axle hub.
To widen the insertion space for the hub bolts within the vehicle, it is conceivable that an insertion angle at which the hub bolt can be inserted into the through-hole of the axle hub is enlarged. To enlarge the insertion angle of the hub bolt to the through-hole, it is necessary to enlarge or cut opening edges of the through-hole at both sides. For this purpose, the opening edges of the through-hole at both sides are chamfered.
However, if the entire periphery of each of the opening edges of the through hole is chamfered to be wide enough to enlarge the insertion angle of the hub bolt, a fitting area between the inner peripheral wall of the through-hole and the outer peripheral portion of the hub bolt (the serrated neck portion) where the hub bolt is fitted to the axle hub is considerably reduced due to the length of the chamfered portions of the through hole. Since the fitting area between the through-hole and the hub bolt is considerably reduced in such a case, the hub bolt is likely to be loosely rotated, together with the nut, relative to the axle hub when the nut is tightened to the hub bolt. Therefore, it is difficult to avoid the lowering of the self-locking resistance of the hub bolt to the axle hub if the chamfering of the opening edges of the through-hole is performed to enlarge the insertion angle of the hub bolt.
Accordingly, in the case of the structure of the above publication, it is difficult to facilitate the replacement of the bolt of the supporting member as well as avoid the lowering of the self-locking resistance of the bolt to the supporting member below the tightening torque of the nut.